The present invention relates generally to a process for preparing radiation sensitive positive photoresist compositions and particularly to compositions containing novolak resins together with naphthoquinone diazide sensitizing agents. It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The novolak resin component of these photoresist formulations is soluble in alkaline aqueous solution, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subject to treatment by a substrate-etchant solution or gas plasma. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which, in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of photoresist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Increased photospeed is important for a photoresist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as, in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, increased photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. These optimum conditions include a constant development temperature and time in a particular development mode, and a developer system selected to provide complete development of exposed resist areas while maintaining a maximum unexposed resist film thickness loss not exceeding about 10 percent of initial thickness.
Development contrast refers to a comparison between the percentage of film loss in the exposed area of development with the percentage of film loss on the unexposed area. Ordinarily, development of an exposed resist coated substrate is continued until the coating on the exposed area is substantially completely dissolved away and thus, development contrast can be determined simply by measuring the percentage of the film coating loss in the unexposed areas when the exposed coating areas are removed entirely.
Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist. Although negative photoresists, wherein the exposed areas of resist coating become insoluble and the unexposed areas are dissolved away by the developer, have been extensively used for this purpose by the semiconductor industry, positive photoresists have inherently higher resolution and are utilized as replacements for the negative resists.
A problem with the use of conventional positive photoresists in the production of miniaturized integrated circuit components is that the positive resists generally have slower photospeed than the negative resists.
Various attempts have been made in the prior art to improve the photospeed of positive photoresist compositions. For example, in the U.S. Pat. No. 3,666,473, a mixture of two phenolformaldehyde novolak resins was utilized together with a typical sensitizer, said novolak resins being defined by their solubility rates in alkaline solutions of a particular pH and by their cloud points. In U.S. Pat. No. 4,115,128, a third component consisting of an organic acid cyclic anhydride was added to the phenolic resin and naphthoquinone diazide sensitizer to provide increased photospeed.
The present invention provides a process for producing an improved positive working photoresist which demonstrates substantially increased photospeed while retaining or improving the resist erosion rate, plasma etch rate, and contrast. It has been unexpectedly found that such an improved resist can be formulated when the novolak resin and quinone diazide photosensitizer are blended with a propylene glycol alkyl ether acetate component. This component also acts as a solvent for the resin and photosensitizer to facilitate the application of the resist to a substrate. This component demonstrates lower toxicity over other solvents useful for forming photoresists.